A tank-venting system typically has the following components: a tank having a tank closure device; an adsorption filter with a venting line; a tank-venting valve; a tank connecting line between the tank and the adsorption filter; and, a valve line between-the adsorption filter and the tank-venting valve.
The tank-venting valve is connected to the intake pipe of an internal combustion engine so that fuel vapors are drawn out of the tank-venting system with the aid of the underpressure in the intake pipe. During this process, the adsorption filter is regenerated with the aid of air which is drawn in via the venting line.
There is the danger that tank-venting systems develop leaks or that obstructions arise. Such systems therefore have to be repeatedly checked as to operability during the operation of a motor vehicle.
The most important method for checking the operability of a motor vehicle tank-venting system is based on a proposal of the California Environmental Authority CARB. According to this method, when opening the tank-venting valve, a check is made as to whether a lambda controller has to carry out a correction of its control output. This is always the case when air with fuel vapor is evacuated by suction out of the tank-venting system. However, it is also the case that the adsorption filter is completely regenerated and that the fuel in the tank is completely evaporated. When the tank-venting valve is opened, no fuel is supplied in addition to that which is supplied to the injection valves of the internal combustion engine according to the control output of the lambda control. In such a case, in which therefore no fuel is supplied by the tank-venting system, that is, the lambda controller does not have to carry out a correction, it is unclear whether the tank-venting system has developed a leak or whether no fuel is being supplied because of the mentioned reasons. In order to be able to decide this question, according to the known method, an evaluation of the signal from the lambda controller only occurs when a fuel temperature sensor indicates that a predetermined minimum fuel temperature is exceeded and a tank fill-level sensor indicates that the vehicle tank has been filled. It is assumed that fuel vapor would then have to be present in the system in any event which vapor is drawn in when the tank-venting valve is opened and which then leads to a correction of the lambda controller. However, with this method incorrect decisions repeatedly occur if there is in fact evaporated fuel in the tank, refilling occurs with the same kind of fuel and the adsorption filter is largely regenerated.
A tank-venting system is described in U.S. Pat. No. 5,193,512 which includes a controllable shut-off valve in the venting line of the adsorption filter. This shut-off valve makes it possible to carry out a method wherein the shut-off valve is closed, the tank-venting valve is opened and then a check is made as to whether an underpressure is building up in the tank. If this is the case, it is concluded that the system is operable.
Erroneous decisions can be made in the above-mentioned method when the fuel vaporizes with great intensity. Furthermore, it is necessary to carry out a special test cycle with the shut-off valve closed wherein the adsorption filter cannot be regenerated.
Accordingly, the problem is presented to provide an especially reliable method for checking the operability of the tank-venting system for a motor vehicle as well as to provide an arrangement for carrying out such a method and a tank-venting system having an operability which can be checked both reliably and to a great extent.